Polymeric materials have a number of advantages over "natural" materials. Generally they are less prone to degradation. While this is an advantage in many applications it is also a disadvantage in some applications. Particularly in packaging applications for food stuffs (i.e. the urban--wilderness camper with the dried food in the plastic packages), or binding applications (i.e. the six pack ring) it would be desirable for the plastic to significantly decompose within a relatively short period of time.
There are however, some other industrial applications where it is also desirable to have plastic material degrade. There are two contrasting applications for plastic sheet film. When plastic material is used as a green house material it should have a fairly long life on the order of at least one year. On the other hand when plastic sheet material is used as a mulch or in row cover applications it is desirable that it lose a significant amount of its tensile strength, preferably at least 50, more preferably from 60 to 80, most preferably more than about 80% loss of tensile properties in from 30 to 160 days.
Plastic mulch is particularly useful in raised bed applications where the bed is prepared and covered with a mulch then subjected to a fluid fumigant or fertilizer. The biggest problem with these applications is that the residual sheet material in the field should be removed before the next preparation of the bed. The old sheet material can get caught in the equipment and cause the new mulch to be improperly put down which can result in a number of problems, including wind flap which cause a loss of young plants.
Canadian patent 1,000,000 sought to address the problem of photodegradability of plastic material by incorporating an unsaturated ketone such as methyl isopropenyl ketone (MIPK) into the plastic. The resulting modified plastic is photodegradable.
U.S. Pat. No. 3,935,141 issued Jan. 27, 1976, to Union Carbide discloses incorporating from 0.002 to 2 weight % (20 to 20,000 ppm) of a cerium salt into polyethylene. The polyethylene must also contain from 0.0025 to 1 weight % of an antioxidant to prolong the life of the polyethylene film. The Union Carbide patent does not suggest that titanium dioxide could also be used in small quantities to prolong the life of the polyethylene film.
At least as early as 1989 Rhone Poulenc published trade literature "Create Your Own Disappearing Act In Plastics With Rhone Poulenc Cerium Stearate". The brochure teaches that polyolefins and polystyrene are photodegraded using cerium stearate. The brochure also teaches on page 2 that cerium stearate can be used with other adjuvants to provide "controlled photo and oxidative degradation of polymeric materials". However, the brochure does not disclose any particular amount or type of adjuvant. Applicant has discovered that titanium dioxide, normally considered an opacifying agent is useful to control the rate of polymer degradation.
U.S. Pat. No. 5,096,941 issued Mar. 17, 1992, from a patent application filed Aug. 23, 1990, assigned to The Dow Chemical Company claims the use of from 0.05 to 0.50 weight % (500 to 5,000 ppm) of cerium +3 stearate to promote the thermo-oxidation and photo-oxidation of polyolefins. The Dow patent does not suggest the use of titanium dioxide in the conjunction with the cerium stearate to help prolong the life of films of polyethylene containing cerium salts.
There are other systems which initiate photodegradation of various polymers such as aromatic compounds or complexes as illustrated by DE 2230790; BE 801695; BE 795406; BE 801 304; BE 795 944; BE 795 405: DE 2 209 254; BE 795 865; DE 2 209 140; DE 2 209 141; DE 2 209 139; and DE 2 209 138, all assigned to BASF. Such aromatic compounds tend to colour the film. In agricultural mulch it may not be desirable to have a darkly coloured film as it may subject the plants to an extreme temperature.
Accordingly there is a need for a system which provides a lightly or white coloured photodegradable sheet material.